Along with the aging of farmers and globalization of farm produce distribution, the labor savings in farm working and a production cost reduction of farm produce have been issues to be solved. In order to solve these issues, for example, in direct planting rice cultivation, a direct seeding method in which seeds are seeded directly into a farm field has become widespread for the purpose of labor savings in rearing of seeding and transplantation. Among them, a technique in which seeds covered with an iron powder are used in order to increase the specific gravity of the seeds has been noted because floating and outflow of seeds in a paddy field are prevented and, in addition, there is a merit that damage by birds is prevented.
In addition, it has also been noted that a bactericidal effect is secondarily obtained by the iron powder cover.
In order to utilize a direct seeding cultivation method by using seeds covered with an iron powder, as described above, it is required that the applied iron powder film is not peeled easily in transportation and seeding steps. This is because if the iron powder film is peeled, the specific gravity of the seed is reduced and the above-described merits are not obtained and, in addition, a peeled film causes plugging in piping and becoming entangled with a rotation mechanism portion in the transportation and seeding steps and a peeled fine iron powder also causes generation of dust. Consequently, it is necessary that peeling of the iron powder film be minimized.
As for a technology to allow an iron powder to adhere to a rice seed surface and solidify, Patent Literature 1 proposes the following technology as a method for manufacturing an iron powder-covered rice seed.
“A method for manufacturing an iron powder-covered rice seed, characterized by comprising the steps of adding an iron powder and 0.5% to 2% of sulfate (where calcium sulfate is excluded) and/or chloride on a mass ratio relative to the iron powder basis to rice seeds, performing granulation after further addition of water, allowing the iron powder to adhere to the rice seed with rust produced by supplying water and oxygen and inducing an oxidation reaction of a metal iron powder and solidify and, thereafter, performing drying.” (refer to Claim 1 in Patent Literature 1)
In the invention described in Patent Literature 1, rice seeds are seeded using a power spreader or a seeding machine, so that strength characteristics at the level of not being flaked by a mechanical impact is required. Therefore, it is ascertained that the coating has practical strength on the basis of a measurement by a method for measuring a level of flaking of the coating (hereafter referred to as a coating flaking test), i.e. a method in which a mechanical impact is given by dropping the resulting coated rice seed five times from a height of 1.3 into a steel sheet having a thickness of 3 mm.
Meanwhile, according to Patent Literature 1, in the case where iron powders having particle size distributions shown in Table 1 described below are used for coatings, all of samples can maintain practical impact strength in the above-described flaking test of iron powder-covered rice seed, although the iron powder particle size distribution is not specifically noted.
TABLE 1Particle size distribution (percent by mass)more thanmore thanmore thanmore thanmore than45 μm and63 μm and75 μm and106 μm and150 μm andType of iron45 μm or63 μm or75 μm or106 μm or150 μm or180 μm orpowderlesslesslesslesslesslessDSP31785150000270M-20034.143.214.66.11.10.9DNC-30085105000Atomized180 μm or less (unspecified)